Engines are usually designed with the ability to deliver a peak output, although most engine operation is performed well below this peak value. As such, it can be beneficial to operate with some cylinders inducting air without fuel injection as described in U.S. Pat. No. 6,568,177. The approach described in U.S. Pat. No. 6,568,177 also advantageously performs period rich operation in all of the cylinders to reduce stored oxidants (e.g., NOx) in a downstream emission control device the exhaust system.
The inventors herein have recognized, however, that when using such an approach, there can be conditions where such operation results in wasted fuel if there are multiple upstream devices for different cylinder groups. Specifically, fuel can be wasted to reduce stored oxidants in multiple upstream devices to provide rich exhaust gasses to the downstream device. As such, less fuel economy improvement is realized than the theoretical potential. This apparent paradox can be solved by a method for controlling an engine with first and second set of cylinder, where the first set is coupled to a first upstream emission control device, the method comprising:
operating in a first mode with the first set of cylinder combusting an air-fuel mixture that oscillates about stoichiometry and second set of second set of cylinders pumping air without injected fuel; and
after said first mode, operating in a second mode with both sets of cylinders combusting, where the first set of cylinders continues combustion that oscillates about stoichiometry and the second set combusts a rich air-fuel mixture for at least a selected duration, and then combusts an air-fuel mixture that oscillates about stoichiometry.
By using the re-enabled cylinders to provide rich exhaust gasses, several benefits are obtained. First, it is not necessary to deplete the oxygen storage of the first upstream device to re-establish to oxygen storage in the downstream device. And second, enabling combustion rich of stoichiometry allow reliable cylinder re-enablement. Thus, improved emission control can be achieved when ending a partial cylinder cut-out operation. Furthermore, by re-enabling the cylinders at a rich air-fuel ratio, NOx emissions are reduced that could otherwise be emitted since the upstream device coupled to the re-enabled cylinders, as well as the downstream device, can be saturated with oxidants, such as oxygen.
Note that a set of cylinders can include a number of cylinders, such as the cylinder of a bank of the engine. However, the set can include only a single cylinder, as well as some cylinders from a first bank of the engine and some cylinders from a second bank of the engine.